This invention pertains generally to the field of carbon nanotubes and production processes for carbon nanotubes.
Carbon nanotubes are crystalline tubular forms of carbon. They are closely related to the C60 molecules known as fullerenes. The diameters of typical carbon nanotubes range from 10 nanometers (nm) for tubes with a single wall of carbon to 100 nm for tubes with several walls of carbon. Typical tube lengths are from 1 to 10 microns.
Carbon nanotubes have a number of significant potential applications. The high aspect ratio of the nanotubes makes them an ideal candidate for use in nanoprobing. For example, a nanotube having a tip diameter of 10 nm and a length of several microns may be used as the tip of an atomic force microscope to probe deep crevices found on integrated circuits (e.g., a 0.1 micron wide gate surrounded by 2 micron interconnects), nanostructures, and biological molecules. Carbon nanotubes also have exceptional material properties. The mechanical strength of nanotubes are predicted to be two orders of magnitude higher than that of conventional carbon fibers used in carbon-fiber composite materials. The much greater mechanical strength of the nanotubes originates in the crystallinity of the fiber as compared to the amorphous nature of a normal carbon fiber. The utilization of nanotubes in composites now using conventional carbon fibers can thus yield much lighter and stronger composite materials well suited for applications in the defense, aerospace and automobile industries.
The electrical properties of carbon nanotubes are also highly tunable. he tubes can be changed from semimetallic to semiconductive by changing the tube chirality (the handedness of rotation of the molecular structure of the tubes) or by doping the tubes with impurity atoms. Conceivably, nanoscale electronic circuits can be made on a single nanotube.
Nanotubes are typically produced in a carbon arc, similar to that utilized for production of fullerenes. A metal cathode and a carbon anode are initially contacted to start the arc and are then pulled apart to a distance of about 1 mm. The electrodes are typically maintained in an inert gas environment, e.g., helium at 500-700 torr pressure. The high current across the resistivity of the plasma results in heat generation of hundreds of watts in the small volume between the arc electrodes. The cathode and the enclosure for the arc are typically water cooled to avoid damage to these structures. The current densities required to produce nanotubes are typically in the range of 190 A/cm2, with a typical voltage drop across the arc of about 20 volts. For a typical cathode diameter of xe2x85x9 inch, about 250-300 watts of energy is dissipated. This energy is lost thermally via radiation and by melting the carbon on the anode and the cathode. The temperatures in the arc may be 4,000 K or even higher. As a result of these high temperatures, carbon vaporizes at the anode and is carried over to the cathode, where nanotube formation takes place. Over time, a boule of carbonaceous material is formed at the cathode which contains nanotubes. The structure of the boule is highly fractal, that is, it consists of macrobundles which contain microbundles, which in turn contain the nanotubes. To isolate the nanotubes, it is necessary to crush the boule and purify the nanotubes through chemistry, filtration and carbon oxidation. The last process can also be used to thin multi-wall nanotubes.
While for many applications it would be desirable to have nanotubes with lengths of 0.1 mm to 1 mm or greater, the most common nanotube lengths produced using conventional processing are about 1 micron long. In addition, most of the carbon material deposited on the cathode is non-nanotube amorphous carbon material. Consequently, a significant problem with current nanotube production techniques is that the lengths and the yield of nanotubes is very limited, and the cost of production of the nanotubes is very high, significantly limiting the practical applications of the nanotubes produced by conventional processes. Various approaches have been proposed for improving nanotube production, including rotating the cathode and scraping the carbon material off of the rotating cathode, as described in U.S. Pat. No. 5,482,601 to Ohshima, et al. Another approach has been to avoid the problems associated with arcs in gas atmospheres by immersing the arc in liquid nitrogen or other inert liquid as described in U.S. Pat. No. 5,753,088. Significant improvements in production efficiencies and tube lengths are still needed. The major limiting problems in such nanotube arc processes are the excessive production of other non-nanotube carbonaceous materials and arc instabilities. The first factor limits the yield of nanotubes from the production process and the second factor is believed to limit the length of the tubes that are produced.
In accordance with the present invention, the economy and efficiency of production of carbon nanotubes can be significantly increased while allowing for the formation of carbon nanotubes having longer lengths than have been obtained in conventional production processes. The production process may be carried out to increase the relative abundance of carbon nanotubes in the carbon material deposited on the cathode with respect to non-nanotube carbon material as compared to conventional processes, both enhancing the rate of production of carbon nanotubes as well as reducing the effort required to separate the nanotubes from other carbon material that is incidentally produced during the process. Further, the process may be carried out substantially continuously without requiring periodic and labor intensive halting of the production process to remove carbon materials from the electrodes.
The present invention may be carried out utilizing a cathode and a carbon anode mounted together within an enclosure in a conventional manner. An electrical power supply is connected to the anode and cathode to supply arc current. The arc may be established in a conventional manner by contacting the anode and cathode to initiate conduction and then drawing the faces of the anode and cathode away from each other to a satisfactory gap distance while maintaining the arc. In the present invention, a vibrational driver, such as a piezoelectric drive, is coupled to the cathode. In carrying out the present invention, the vibrational driver is activated to vibrate the cathode while the arc is established between the anode and cathode. Under suitable conditions, the cathode may be vibrated at frequencies from about 10 Hz to about 1 MHz or higher. The cathode is preferably vibrated at ultrasonic frequencies, for example, in the range of 10 kHz to 100 kHz. The driver may be operated at the resonant frequency of the cathode and its support to maximize the amplitude of longitudinal mode vibrations (vibrations perpendicular to the plane of the face of the cathode). The vibration of the cathode results in high accelerations of the face of the cathode to dislodge the larger, non-nanotube amorphous particles from the face surface while allowing the lighter carbon nanotubes to remain attached to the surface, thus enhancing the proportion of carbon nanotubes retained on the cathode face as compared to other carbon material. The vibration of the cathode face also results in acoustic streaming to thereby focus the plasma at the face surface. The vibration of the cathode may also be carried out to develop vibrational nodes and antinodes on the face of the cathode, allowing arc current to be focused and stabilized in location at the antinodes of cathode vibration, which enhances the formation of longer nanotubes.
Further in accordance with the invention, substantially all of the particles on the cathode face may be driven off by applying a stress pulse from the driver to the cathode which has a sufficient amplitude to dislodge the entire carbon boule from the face of the cathode. This creates new space for another nanotube boule to be formed. In this manner, continuous carbon nanotube production can be achieved which is limited only by the anode carbon supply, eliminating the need to stop the process to dislodge the carbon from the face of the cathode or to rotate the cathode or otherwise scrape the cathode to remove the carbon boule. Avoiding the need to scrape the cathode also avoids the mechanical damage to the cathode that can be caused by scraping.
The vibrational driver may be connected to a support rod at a position either inside or outside of the arc enclosure. If the driver is connected to the rod outside the enclosure, the support rod then extends through a wall of the enclosure to be connected to the cathode. Vibrations are coupled from the vibrational driver to the support rod and therethrough to the cathode. Coolant may be supplied through the support rod to the cathode to cool the cathode and maintain it at a desired temperature for optimal nanotube production.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.